When the hugely popular San Diego EarthFair started in 1990 to celebrate the 20th anniversary of Earth Day, the region’s environmental quality was a work in progress.

San Diegans were just getting the hang of recycling. They tapped abundant supplies of water at cheap prices, but suffered from dirty air for nearly one out of every four days that year. The nation’s keystone environmental laws had not made a full impact on the county.

The decades since have brought dramatic improvements.

Residents have more than doubled their recycling, cut their per-capita water use by more than a third and seen their region’s level of bad ozone plummet by nearly 97 percent.

Environmental experts said San Diego’s emerging clean-technology industry can help strengthen those gains even more. The county’s economic might and overall quality of life will benefit from reliable, locally sourced water supplies, increased investment in renewable energy and even better ways to keep waste out of landfills, they said.

“We’ve certainly had improvement in a number of areas,” said Lani Lutar, executive director of the Equinox Center in San Diego, a think tank promoting sustainable growth. “Because of the resources we have unique to San Diego and the industries and expertise, it would be foolish of us not to take advantage of that to do better.”

The following is a gauge of the region’s environmental health based on five basic indicators:

Air quality

Longtime San Diegans share stories of itchy eyes and scratchy throats from past exposure to smog. The county’s air isn’t perfect now, but the clear blue skies enjoyed on most days are a clear sign of advancement.

The region failed federal ozone standards on 96 days in 1990, when it had 2.5 million residents and vehicles were on the road for roughly 65 million miles per day. In 2011, the region exceeded the benchmarks on just three days, even though the population had risen to 3.2 million people and 87 million vehicle miles being driven daily.

Most of the progress comes from cleaning up pollution on the road and at the gas pump, said Bill Brick, senior meteorologist for the San Diego County Air Pollution Control District.

Future efforts to further boost air quality will take aim at diesel exhaust from heavy trucks, buses and tugboats, he said.

Bonnie Holmes-Zen, senior director of policy and advocacy for the American Lung Association, said San Diegans still need to drive less and drive cleaner.

“The big, next step we need to take statewide is to move away from dirty, petroleum-based fuels and toward cleaner alternative fuels,” she said.

Water consumption

San Diegans used an average of 237 gallons per day in 1990, for a total of 646,645 acre-feet, according to the San Diego County Water Authority. An acre-foot is the amount needed to cover an acre a foot deep.

Last year, per-capita use had dropped by more than a third, to 151 gallons per day.

Moreover, total use had also fallen by about 17 percent, while population in the water authority’s service area surged from 2.4 million to 3.2 million people.